There is known a humidifier that humidifies cathode gas to be supplied to a fuel cell by use of moisture contained in cathode off-gas discharged from the fuel cell. Desirably, the humidification amount of the cathode gas is suitably adjusted. For example, if the humidification amount of the cathode gas by the humidifier is too large when the temperature of the fuel cell is relatively low, a large amount of condensed water might be generated in the fuel cell, so that flooding might occur. Further, if the humidification amount of the cathode gas is insufficient when the temperature of the fuel cell is relatively high, an electrolyte membrane of the fuel cell might be dried. For example, Japanese Unexamined Patent Application Publication No. 2006-156203 discloses: a bypass path for bypassing cathode off-gas from a humidifier; and a bypass valve for adjusting the bypass amount of the cathode off-gas.
The above-described bypass path and bypass valve are provided outside the humidifier, which might complicate an entire system and might increase a size thereof.